Electromechanical transducer



NOV. 1956 F. R. JOHANSON ELECTROMECHANICAL TRANSDUCER 2 Sheets-Sheet 1 Filed March 8, 1963 INVENTOR. FEEDER/C R. JOHA/VSON BY mug/#5) MILLER 6 RA a0 a giro NEYS a N 1 1966 F. R. JOHANSON 3,283,181

ELECTROMECHANIGAL TRANSDUCER Filed March 8, 1963 2 Sheets-Sheet 2 INVENTOR. F RE DE R/C R JOHA NSON BY MAHON Y, M/ LEI? 8/? M80 G AT ORNEYS United States Patent J aeger Machine Company, Columbus, Ohio, 21 corporation of Ohio Filed Mar. 8, 1963, Ser. No.- 263,959 1 Claim. c1. SID-8.6)

This invention relates, generally, to an electromechanical transducer. It relates, more specifically to a novel and improved pressure-responsive microphone of the type having a piezoelectric crystal transducer.

A well known type of pressure-responsive microphone that is currently utilized in many applications is of the general construction having a housing or supporting structure in which is mounted a piezoelectric transducer and a flexible diaphragm member. The diaphragm member and the housing form a closed chamber with the fluid contained therein exerting a pressure on the interior surface of the diaphragm. A suitable mechanical interconnection is made between the crystal and the diaphragm for the transmission of distorting forces. Although this type of microphone may be constructed to have a relatively high degree of sensitivity, such as is required for detection of fetal heart sounds, it is responsive to all fluid transmitted pressure Waves that may be produced by any number of physically vibrating or oscillating devices in close proximity to the microphone. All fluid-transmitted pressure waves thus generated will be noted as a differential pressure on the diaphragm and will thus be effective in applying a distorting force to the crystal. For this reason, microphones of this prior type have been found to be unsatisfactory Where it is desired to detect a specific mechanical vibration or oscillation as in the medical diagnostic field Where the interpretation of a patients heart sounds may be of critical importance in determining the correct treatment. In most examination rooms, many other sounds represented as fluid pressure waves are also present and will substantially interfere with the physicians aural ascertainment of a patients heart sounds. Interference of this form will be particu larly noticeable in diagnosis involving fetal heart sounds occurring during the intermediate and final stages of a pregnancy.

It is, therefore, the primary object of this invention to provide a pressure-responsive type microphone which is substantially unresponsive to fluid-transmitted pressure Waves and which has a relatively high degree of sensitivity that is adequate for a medical diagnosis including the discernment of fetal heart sounds.

It is another object of this invention to provide a pressure-responsive type microphone having a piezoelectric crystal transducer supported independently of a diaphragm which is mechanically coupled thereto with the size of each being independently determined for maximum sensitivity and minimum response to fluid-transmitted pressure waves.

It is also an object of this invention to provide a presminimum of associated electronic amplification equipment.

3,283,181 Patented Nov. 1, 1966 These and other objects and advantages of this invention will be readily apparent from the following detailed description and the accompanying drawings illustrating an embodiment thereof.

In the drawings:

FIGURE 1 is a pictorial representation of the utilization of the microphone of this invention.

FIGURE 2 is an enlarged plan view of the back of the microphone.

FIGURE 3 is a medial sectional view taken along line 3-3 of FIGURE 2.

FIGURE 4 is a transverse sectional view taken along line 4-4 of FIGURE 3.

FIGURE 5 is a fragmentary transverse sectional view taken along line 5-5 of FIGURE 3.

Referring to the drawings, an embodiment of the microphone of this invention is illustrated in FIGURE 1 as it may be utilized in medical diagnosis. The microphone 10 is maintained in the appropriate position in contact with the patients body by a strap 11, or other suitable means, and is connected by an electrical cable 12 to an electronic amplification apparatus 13. The amplification apparatus 13 is illustrated as including a recording device although this may be eliminated and an electronic amplifier of the miniaturized type may be substituted provided it is capable of providing an adequate power output. For aural interpretation by the physician, the amplified electrical signal may be converted to sound waves by another electromechanical transducer. In the illustrated example, the transducer 14 is of a type which is adapted to be utilized with a conventional stethoscope 15 in place of the usual chest piece. Alternatively, the transducer 14 and stethoscope 15 may be replaced by a conventional audio loudspeaker that is driven by the amplifier 13. Also, the microphone 10 may be combined with other apparatus and equipment such as constructing an integral microphone, amplifier and electromechanical transducer adapted to be connected to aconventional stethoscope. The microphone 10 is illustrated as comprising a case or housing 16 adapted to be connected to the strap 11 by means of a pair of opposed, laterally-extending ears or tabs 17. Attached to the marginal ends of each tab 17 is a post 18.having an enlarged head portion releasably engageable with one of a plurality of spaced holes formed in the strap. The housing 16 is of cylindrical, tubular form, preferably fabricated from an electrically-conducting metal, having an end wall 19 integrally formed therewith at one end. Adapted to be fixed in the opposite open end or back of the housing 16 is a closure member or retainer 20. The retainer 20 consists of a flat disc adapted to be disposed in the open end of the housing and is maintained in position by an interference fit or other suitable fastening means. Formed in the central portion of the retainer 20 is an aperture 21 through which the cable 12 may extend. Secured to the exterior surface of the retainer 20 is a cup-shaped cap 22 for attachment of the cable 12 -to the microphone and relief of mechanical strain on the conductor-terminal connections. A pair of cap screws 23 extend through the cap 22 and are threaded into the retainer 20. The cable 12 preferably extends through an opening formed in the cylindrical wall of the cap 22 and is retained in fixed position by a metal grommet 2-4 which is of a type that securely grips the cable and is maintained in tixed relationship to the cap. The cable 12 is thus attached to the microphone normal to the longitudinal axis of the housing 16 to prevent transmission of forces through the cable that .are axially aligned with the housing.

Disposed within the interior of the housing 16 is a piezoelectric crystal supporting structure comprising a cylindrical cup 25 of rigid construction. The crystal cup 25 is positioned with the open end adjacent the end wall 19 of the housing with a lip portion thereof in contacting engagement with the interior cylindrical wall of the housing to maintain axial alignment. A pair of spacing members 26 extending partially around the housing are disposed between the edge of the lip portion and the end wall 19. The end wall 19 may be formed with an annular ledge 19a as indicated on which the spacing members 26 are supported in opposed relationship. Formed on the exterior surface of the base of the crystal cup 25 is an upstanding post 27 which is positioned thereon to engage the interior surface of the retainer 20. In the assembly of the microphone, the crystal cup 25 is first positioned within the housing 16 as indicated and the retainer pressed into the open end in contacting engagement with the post 27. Thus, the crystal cup will be maintained in .a fixed position within the housing with a space provided between the base of the cup and the retainer 20 for attachment of the cable 12 to a terminal 28 carried by the cup but electrically insulated therefrom.

A suitable piezoelectric crystal 30 is appropriately mounted on the interior of the base of the crystal cup 25 by a bifurcated arm type, spring contact, clamp member 31. The clamp member is electrically and mechanically attached to the terminal 28 and completes a circuit to a ribbon lead 32 of the crystal. A pair of ribs 33 and 34 are integrally formed on the interior surface of the cup base on which the crystall 30 is supported. The two ribs 33 and 34 are in quadrature and support the crystal on two adjacent marginal edges a distance from the base of the cup and in a plane parallel thereto. For the most eflicient production of an electrical signal by a crystal of this mounting type, a distorting force is applied to the crystal 30 at the unsupported corner in a direction normal to the plane of the crystal and parallel to the longitudinal axis of the housing by means of a suitable connector 35. A second ribbon lead 36 connected to the crystal is grounded to the cup 25 by being clamped between the crystal and the rib 34. The cable 12 is of the two-conductor type, preferably coaxial, with one conductor connected to the terminal 28 and the other grounded to the housing. Assembly of the microphone is facilitated by positioning the crystal to align the connector 35, which includes an elongated shaft portion, with the central longitudinal axis of the housing 16.

In accordance with this invention, a diaphragm member 37 of elongated bar shape is provided for the reception of distorting forces and the subsequent transmission of a proportional force to the crystal 30. The diaphragm 37 is formed from a material having the desired resilience characteristics. For example, the material may consist of an epoxy impregnated fiber glass molded into sheet stock of the desired thickness. The diaphragm 37 is of a thickness substantially equal to the thickness of the spacing members 26 and is positioned in the lions ing 16 with the marginal ends thereof disposed between silience of the diaphragm 37.

ported on both the spacers 26 and the diapragm 37, there will be no tendency for the cup to rock or rotate about the longitudinal axis of the diaphragm and subject the crystal to damaging stresses.

An adaptor 38 is mounted centrally of the diaphragm 37 for mechanically coupling the diaphragm to the crystal 30. The adaptor 38 comprises an elongated threaded stud disposed perpendicular to the plane of the diaphragm in axial alignment with the longitudinal axis of the housing and extending a distance to either side of the diaphragm. A pair of nuts 39 are threaded on the adaptor 38 to clamp the diaphragm therebetween in fixed relationship. The portion of the adaptor 38 extending inwardly of the housing is formed with an axially disposed central bore for receiving an end of the crystal connector 35. A rigid connection may be formed between the adaptor 38 and connector 35 by a suitable adhesive bonding agent.

An aperture 43 is formed in the end wall 19 in coaxial alignment with the adaptor 38 to permit transmission of distorting forces to the diaphragm 37. Threadedon the end of the adaptor 38 adjacent the aperture 43 is a contacting member 40 which is adapted to engage the patients body. The contacting member 40 consists of a hub portion 41 which is formed with a threaded axial bore, and an enlarged head portion 42 of disc shape.

Distorting forces applied to the contacting member 40 will be transmitted to the crystal 30 through the diaphragm 37. The amplitude of the transmitted distorting forces, represented by axial displacement of the connector 35, is proportional to the distorting force applied to the contacting member 40 and is a function of the re- Sensitivity is therefore determined by the diaphragm 37 and the size of the crystal 30 and may be increased or decreased as desired by inversely varying the width of the diaphragm and directly varying the size of the crystal 30.

Only distorting forces produced by physical movement of an object in contacting engagement with the contacting member 40 will be effective in distorting the crystal 30 and produce a signal. Fluid-transmitted pressure Waves will not produce a distorting force on the diaphragm 37 or the contacting member 40 as the pressure waves will also be reflected against the reverse side of each of the'cornponents. The particular cable attachment previously described further aids in the reduction of extraneous distorting forces and consequent interfering signals. Mechanical stresses are transmitted axially of a flexible cable and the stresses applied to the cable 12 are transferred to the housing 16 by the metal grommet 24. However, the stresses thus transmitted are exerted along an axis in quadrature to the axis of the crystal along which the distorting forces must be applied to be effective in producing a signal and will therefore not subject the crystal to such extraneous distorting forces.

It is readily apparent that the novel pressure-responsive microphone of this invention is particularly adapted to the detection of very small amplitude mechanical vibrations and oscillations. Construction of the diaphragm as an elongated bar which does not form a closed chant ber with the associated supporting structure thus sub the ledge 19a of the end wall and the lip portion of the crystal cup 25. Maintenance of the diaphragm 37 in its proper position is thus accomplished by the crystal cup 25 in association with-the retainer 20. Flexing movement of the diaphragm 37 axially relative to the housing 16 in either direction is unrestricted as the ledge 19a supports the diaphragm a distance from the interior surface of the end wall 19. with the crystal cup 25- thus supstantially eliminates any response of the crystal to fluidtransmitted pressure waves which represent interference signals. Sensitivity of the microphone may be increased to permit detection of fetal heart sounds during the intermediate and final stages of a pregnancy.

According to the provisions of the patent statutes, the principles of this invention have been explained and have been illustrated and described in what is now considered to represent the best embodiment. However, it is to be understood that, within the scope of the appended claim, the invention may be practiced otherwise than as sepcifically illustrated and described.

Having thus described this invention, what is claimed is:

An electromechanical transducer comprising a housing, a piezoelectric crystal disposed within said housing and supported for displacement about a voltage-produc' ing axis, said crystal having an electrical cable connected thereto which extends exteriorly of said housing through an opening formed in said housing with said opening being arranged to maintain said cable substantially normal to the axis of motion of said crystal, and a diaphragm of elongated bar form supported in a normally unfiexed configuration within said housing for flexing movement in a plane parallel to the displacement axis of said crystal,

said diaphragm being supported at each end and per mitting the relatively free flow of fluid therearound.

References Cited by the Examiner 5 UNITED STATES PATENTS 2,175,021 10/1939 Gibbs 3108.6 2,561,084 7/1951 Wickharn et a1. 1791l0.1 X 2,702,354 2/1955 Chorpening 179-110.1

l0 MILTON O. HIRSHFIELD, Primary Examiner.

ROBERT H. ROSE, Examiner.

A. J. ROSSI, R. LINN, Assistant Examiners. 

